Green paradox: planting trees will cool a megacity unless it’s dry

Cities around the world are getting hotter for two reasons: the climate is warming and urban areas often trap heat more than the countryside. Planting more vegetation, especially trees, has become a popular ‘nature-based’ way to cool cities. But how much does this really help?

To answer this, researchers from Australia, China, Saudi Arabia, and Switzerland recently compared the temperature over different kinds of urban land cover, including trees, grasslands, croplands, and built-up surfaces like concrete and asphalt, in 761 megacities in 105 countries worldwide, including India. They defined a measure called temperature regulation capability: the temperature of a vegetated area minus the temperature of a built-up area. If the number was negative, vegetation was cooler, and vice versa.

When they analysed the data, the researchers found a paradox. In many cities, vegetation cooled, but in dry places, it could warm. Across all cities, grasslands cooled built-up areas in 78% of cases and trees cooled in 98%. But in almost a fourth of cities, especially in places with under 1,000 mm of rain a year, urban grasslands and croplands were hotter than built-up areas, creating net warming. Even trees showed warming in 2% of arid cities.

The researchers published their findings in Science Advances on January 2.

They used a combination of physical effects to explain the paradox. Vegetation can cool a surface by evapotranspiration, i.e. water evaporating from soil and transpiring from leaves, carrying heat away. But vegetation can also absorb more sunlight if it reflects less light than some built surfaces. In arid cities, the cooling weakens because water is scarce, so evapotranspiration is limited. Then the warming could ‘win’, i.e. reflection-driven warming plus changes in stored heat could outweigh the weaker cooling.

The authors also examined what happened during extremely hot summers (months hotter than the 85th percentile of a long-term average). In about 75% of cities, trees reduced how much the temperature rose compared to built-up areas. Grasslands and croplands often did the opposite, worsening heat increase in about 71% and 82% of cities. One reason was that extreme heat often came with a large deficit in the vapour pressure, which caused many grasses and crops to shut down water loss more strongly, reducing cooling from evapotranspiration.

As the authors concluded, planting trees is not a simple solution and “misguided greening risks are worsening urban warming”.